The invention relates to casters in general, and more particularly to improvements in casters which can be utilized with advantage on the legs of hospital beds or for similar purposes. Still more particularly, the invention relates to improvements in casters of the type disclosed in commonly owned copending patent application Ser. No. 07/768,059 filed Sep. 27, 1991 for "Caster with pivotable two-armed wheel brake" now U.S. Pat. No. 5,242,035.
The commonly owned copending patent application Ser. No. 07/768,059 discloses a caster wherein a post which is reciprocable in an upright support can be moved up and down between three different positions by a rotary cam which is installed in the support or in the leg of a bed or the like. When the post is moved to one of the three positions, a pivotable braking device not only holds the wheel of the caster against rotation relative to the frame but also prevents the frame from swiveling or turning relative to the support. If the post is moved to another axial position relative to its support, two projections which are provided on a disc-shaped portion of the post enter two complementary recesses in a part of the wheel frame to ensure that the frame is held against swiveling relative to the post (i.e., relative to the support for the post) but the brake is disengaged so that the wheel is free to turn about its axis. This ensures that the direction of forward or rearward movement of the caster remains unchanged. The projections are urged into the respective recesses by a spring which further serves to urge the post toward engagement with the cam. As a rule, the projections have a substantially rectangular or square cross-sectional outline and are snugly receivable in complementary recesses of the wheel frame. This ensures that the orientation of the caster wheel cannot be changed unless the post is forcibly shifted to a different axial position, namely a third position in which the projections are expelled from their recesses and the brake is still disengaged so that the wheel can turn in the frame and the frame is free to swivel in the support.
The projections could be expelled from their recesses by imparting thereto a substantially triangular, trapezoidal or like shape. This would enable an operator to expel the projections from their recesses in response to the application to the wheel frame of a force which is sufficiently pronounced to ensure that the tapering projections must leave their recesses in order to permit a change of angular position of the wheel frame. A drawback of such design is that the person in charge of applying the required force must overcome a substantial amount of friction which, in turn, causes pronounced wear upon the parts to be moved relative to each other. Moreover, the person in charge is not apprised of the exact instant when the wheel frame becomes free to turn or swivel relative to the support. In addition, the bias of the spring which opposes axial displacements of the post and of the wheel frame cannot be adjusted with a required degree of precision, and the bias of such spring is often sufficiently pronounced to prevent a person of average strength from changing the orientation of the wheel frame except by actuating the cam and by thus moving the post to a different axial position in the support prior to actual start of a change in the angular position of the frame. On the other hand, it is equally undesirable to employ a relatively weak spring which is ready to yield in reply to the application to the wheel frame of a small turning force because this would enable the frame to change its orientation at times when the angular position of the frame should remain unchanged. Accordingly, there exists a need for a caster which is constructed and assembled in such a way that the magnitude of the force to be applied in order to change the orientation of the wheel frame can be selected in advance and that the magnitude of such force be variable within a reasonably wide range without affecting the reliability of stability of the frame in a selected angular position relative to its support.